1000 a. service ground fault protection

[realolman]

I wonder if someone would please explain the rationale re: art. 230.95 requiring ground fault protection of grounded wye services rated 1000 a. or more.


thanks

 

[Pierre C Belarge]

1000 amp, 480 volt system.

The 480 volt system will perpetuate the faulting condition. In the service size of 1000 amp and larger, I guess they decided that was the size the requirement would be started at...I believe there is substantial documentation of fires from these systems, starting back in the '50s

 

[chris kennedy]

The commentary following 230.95 in the handbook states "Ground-fault protection of equipment on services rated 1000 amps or more operating at 480Y/277 volts was first required in the 1971 Code because of the unusually high number of burndowns reported on those types of services."

 

[brian john]

There is a thread somewhere in the post, that explains th rational. In higher voltage distribution systems an sustained arcing ground his an impedance that is below the thresholds of OCP's, the sustained arcing ground faluts can burn to the point where the ground fault current exceeds the OCP rating and opens the device or becomes a phase to phase faultgenerating sufficent current to operate the OCP device.


I located the thread.

http://forums.mikeholt.com/showthread.php?t=83034&highlight=ground+fault

From that thread


Overcurrents and Undercurrents All About GFCIs?.
Author Earl W. Roberts,
1996, Mystic Publications

This book also covers GFPE, from chapter 12 (my abridged version)

While arcing faults can occur on 120/208 VAC systems the arc does not result in a restrike arc due to the voltage level. Studies have shown that to have a restriking arcing ground fault the voltage levels of 374 volts are necessary.

120X1.414 = 169.68, Peak Voltage
277X1.414 = 391.67. Peak Voltage

GFPE is required on 277/480 VAC distribution systems to protect against the devastating damage that occurs from these sustained restriking arcing ground faults, it is not required on 120/208 distribution systems as the restriking high impedance burn down arcing ground faults are not occurring.

 

[bphgravity]

Overcurrents and Undercurrents is a great book. I purchased it a few years back and read it over and over quite often. It's also great reference for answering questions on this and other electrical forums. I suggest everyone get himself or herself a copy.